LOW LEAKAGE ReRAM FPGA CONFIGURATION CELL

ABSTRACT

A low-leakage resistive random access memory cell includes a complementary pair of bit lines and a switch node. A first ReRAM device is connected to a first one of the bit lines. A p-channel transistor has a source connected to the ReRAM device, a drain connected to the switch node, and a gate connected to a bias potential. A second ReRAM device is connected to a second one of the bit lines. An n-channel transistor has a source connected to the ReRAM device a drain connected to the switch node, and a gate connected to a bias potential.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/268,704 filed Dec. 17, 2015, the contents of whichare incorporated in this disclosure by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to semiconductor technology. Moreparticularly, the present invention relates to memory cell technologyand to resistive random access memory cell technology. The presentinvention relates to low leakage resistive random access memory cells.

The contents of co-pending applications attorney docket no. 7618-52197-1entitled LOW LEAKAGE RESISTIVE RANDOM ACCESS MEMORY CELLS AND PROCESSESFOR FRABRICATING SAME; attorney docket no. 7618-52597-1 entitledTHREE-TRANSISTOR RESISTIVE RANDOM ACCESS MEMORY CELLS; and attorneydocket no. 7618-52666-1 entitled THREE-TRANSISTOR RESISTIVE RANDOMACCESS MEMORY CELLS filed on the same date of this application areexpressly incorporated herein by reference in their entirety.

2. The Prior Art

Resistive random access memory (ReRAM) push-pull memory cells make anattractive configuration memory for advanced field-programmable gatearray (FPGA) integrated circuits due to their small size andscalability. Examples of ReRAM memory devices and. memory cellsconfigured from those devices are disclosed in U.S. Pat. No. 8,415,650.

A ReRAM device is basically two metal plates, one of which serves as ametal ion source, separated by a solid electrolyte. The solidelectrolyte has two states. In a first state (an “on” state), ions fromthe metal ion source have been forced into the electrolyte by placing aDC voltage having a first polarity across the ReRAM device and having asufficient potential to drive metal ions from the ion-source plate intothe electrolyte. In the first state, the ions form a conductive bridgethrough the solid electrolyte across which electrons can pass fairlyeasily. As the electrolyte becomes increasingly populated with metalions, its resistivity, and hence the resistivity of the entire ReRAMdevice decreases. In a second state (an “off state), the electrolyte hasbeen virtually depleted of ions by placing a DC voltage having apolarity opposite to that of the first potential and a potentialsufficient to drive the metal ions from the electrolyte back into theion-source plate across the ReRAM device, In the second state, absenceof the ions makes it difficult for electrons to pass through the solidelectrolyte. As the population of metal ions in the electrolytedecreases, its resistivity, and hence the resistivity of the entireReRAM device increases. Amorphous silicon is a solid electrolyte and itis a leading candidate today for use in ReRAM devices.

ReRAM devices are often employed in a push-pull configuration to form aReRAM memory cell as shown in FIG. 1. ReRAM memory cell 10 includes afirst ReRAM device 12 in series with a second ReRAM device 14. In theReRAM device symbols shown in FIG. 1, the wider (bottom) end of theReRAM device is the end nearest its ion source. A voltage applied acrossthe ReRAM device with its positive potential at the narrower (top) endof the ReRAM device will erase the device, i.e., set it to its “off”state, and a voltage applied across the ReRAM device with its positivepotential at the wider (bottom) end of the ReRAM device will program thedevice, i.e., set it to its “on” state.

The ReRAM devices 12 and 14 are connected in series between a pair ofcomplementary bitlines (BL) 16 (BL!) 18. Persons of ordinary skill inthe art will appreciate that the value of the potentials applied to (BL)16 and (BL!) 18 will be selected as a function of the particular featuresize and other aspects of the technology employed. Typical operatingvoltages that are applied to (BL) 16 and (BL!) 18 are 1.5V and 0V,respectively.

In operation, one of ReRAM devices 12 and 14 will be set to its “on”state and the other will be set to its “off” state. Depending on whichone of the ReRAM devices 12 and 14 is “on” and which one is “off” switchnode 20 will either be pulled up to the voltage on BL 16 or pulled downto the voltage on BL! 18.

The gate of a switch transistor 22 is coupled to a switch node 20. Thedrain of the switch transistor 22 is connected to a first programmablenode 24 and the source of the switch transistor is connected to a secondprogrammable node 26. The first programmable node 24 can be connected tothe second programmable node 26 by turning on the switch transistor 22.

If ReRAM device 12 is in its “on” state and ReRAM device 14 is in its“off” state, switch node 20 is pulled up to the voltage on BL 16, andswitch transistor 22 will be turned on. If ReRAM device 12 is in its“off” state and ReRAM device 14 is in its “on” state, switch node 20 ispulled down to the voltage on BL! 18, and switch transistor 22 will beturned off. Persons of ordinary skill in the art will note that theentire potential between (BL) 16 and (BL!) 18 will exist across the oneof ReRAM devices 12 and 14 that is in the “off” state.

A programming transistor 28 has a gate coupled to a word line (WL) 30.The drain of programming transistor 28 is connected to switch node 20and its source is connected to a word line source (WLS) 32. In a typicalapplication, ReRAM devices 12 and 14 are first erased (set to their“off” state) and then one of them is programmed (set to its “on” state)as described herein with reference to FIG. 5.

Referring now to FIG. 2, a cross sectional view of an illustrativesemiconductor layout for a ReRAM cell 10 like that of FIG. 1 is shown.The ReRAM cell 10 is shown formed in a p-type semiconductor substrate34, which may be a p-well structure as is known in the art. Shallowtrench isolation (STI) regions 36 separate active regions for the switchtransistor, the programming transistor and other adjacent structures.N-type doped region 38 forms the drain of the programming transistor andn-type region 40 forms its source. A contact 42 connects source 40 ofthe programming transistor to a first segment 44 of a first layer (M1)of metal interconnect forming WLS 32 described above. Polysilicon line46 forms the gate of the programming transistor 28 and also acts as wordline WL 30. Persons of ordinary skill in the art will appreciate thatn-type region 40 can also serve as the source of a programmingtransistor for an adjacent ReRAM cell configured in a mirror cellarrangement with ReRAM memory cell 10 as is known in the art.

The switch transistor 22 is oriented orthogonally to the programmingtransistor 28 and polysilicon line 48 forms its gate. The source 26 anddrain 24 regions of the switch transistor 22 are located in planesbehind and in front of the plane of FIG. 2. Region 50 under thepolysilicon line 48 is the channel of the switch transistor 22.

ReRAM device 12 is formed between a second segment 52 of the first layer(M1) of metal interconnect and a first segment 54 of a second layer (M2)of metal interconnect. An inter-metal contact 56 is shown connectingReRAM device 12 to the first segment 54 of the second layer (M2) ofmetal interconnect. A second segment 58 of the second layer (M2) ofmetal interconnect serves as the bitline BL 16 and is connected to thesecond segment 52 of the first layer (M1) of metal interconnect by aninter-metal contact 60.

ReRAM device 14 is formed between a third segment 62 of the first layer(M1) of metal interconnect and a third segment 64 of the second layer(M2) of metal interconnect. The third segment 64 of the second layer(M2) of metal interconnect serves as the bitline BL! 18. An inter-metalcontact 66 is shown connecting ReRAM device 14 to the third segment 64of the second layer (M2) of metal interconnect.

An inter-metal contact 68 between the first segment 54 of the secondlayer (M2) of metal interconnect and the third segment 62 of the firstlayer (M1) of metal interconnect is used to make the connection betweenReRAM device 12 and ReRAM device 14. Another pair of inter-metalcontacts 70 and 72 and the third segment 62 of the first layer (M1) ofmetal interconnect are used to make the connection between the gate 48of the switch transistor, the drain 38 of the programming transistor 28,and the common connection of the ReRAM devices 12 and 14.

ReRAM devices in the “off” state do not exhibit infinite resistance.ReRAM devices will therefore pass a leakage current in the “off” stateif a voltage is impressed across them. For most normal memoryapplications, bits are only read when they are addressed. A transistormay be used to block any leakage current during times when the bit isnot being read, and the leakage is therefore not overly problematic.

However, when using a ReRAM cell as a configuration memory for an FPGA,the cell statically drives the gate of a switch transistor to place theswitch transistor in either its “on” or “off” state. In thisapplication, the ReRAM cell is essentially always being read. Thus, theleakage current is always present across the ReRAM device that is in the“off” state, if a voltage is impressed thereacross, and is problematic.

Current investigations of the use of ReRAM memory cells as configurationmemory in FPGA integrated circuits are academic in nature and ignore thecell leakage issue which presents a practical problem inhibiting thecommercial application of this technology. Because the amount of “off”state leakage is an exponential function of the reverse, or “off” state,voltage across the ReRAM device it presents a significant obstacle tocommercialization of configuration memory in FPGA devices.

BRIEF DESCRIPTION

According to the present invention a low-leakage resistive random accessmemory cell includes a complementary pair of bit lines and a switchnode. A first ReRAM device is connected to a first one of the bit lines.A p-channel transistor has a source connected to the ReRAM device, adrain connected to the switch node, and a gate connected to a biaspotential. A second ReRAM device is connected to a second one of the bitlines. An n-channel transistor has a source connected to the ReRAMdevice a drain connected to the switch node, and a gate connected to abias potential. A programming transistor has a drain connected to theswitch node, a source connected to a source word line and a gateconnected to a word line. A switch transistor has a gate connected tothe switch node, a source connected to a first programmable node and adrain connected to a second programmable node.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a schematic diagram of an illustrative prior-art push-pullReRAM cell.

FIG. 2 is a cross sectional view of an illustrative semiconductor layoutfor a ReRAM cell like that shown in FIG. 1.

FIG. 3A is a schematic diagram of an illustrative push-pull ReRAM cellaccordance with the present invention.

FIG. 3B is a plot showing the current vs. voltage characteristics of anoff-state ReRAM device and a MOS transistor operating in subthresholdregion.

FIG. 4A is a cross-sectional view of an illustrative semiconductorlayout for a ReRAM cell like that shown in FIG. 3A.

FIG. 4B is a top view of an illustrative semiconductor layout for aReRAM cell like that shown in FIG. 3A.

FIG. 5 is a schematic diagram depicting four illustrative ReRAM cells inan array to show a method for programming and erasing the ReRAM cells.

FIG. 6 is a table showing voltages to be applied to the ReRAM memoryarray of FIG. 5 to erase and program the cells.

FIG. 7A is a schematic diagram showing another illustrative embodimentof a push-pull ReRAM cell in accordance with the present invention.

FIG. 7B is a schematic diagram showing a variation of the programmingtransistor portion of the push-pull ReRAM cell of FIG. 7A.

FIG. 7C is a schematic diagram showing another variation of theprogramming transistor portion of the push-pull ReRAM cell of FIG. 7A.

FIG. 7D is a schematic diagram showing yet another variation of theprogramming transistor portion of the push-pull ReRAM cell of FIG. 7A.

DETAILED DESCRIPTION

Persons of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons.

By adding two transistors to the cell in accordance with the presentinvention, the voltage across the “off” state ReRAM cell can besignificantly reduced (e.g., to 0.5V or less) while maintaining the 1.5Vdrive on the switch transistor in a typical application as describedherein. This can be done with only a minimal increase in area. Becausethe leakage is an exponential function of applied voltage, reducing thevoltage across the “off” state ReRAM device in accordance with thepresent invention dramatically reduces the leakage through the “off”state ReRAM device, significantly improving the power dissipation.

Referring now to FIG. 3A, a schematic diagram shows an illustrativepush-pull ReRAM cell 80 in accordance with the present invention. ReRAMmemory cell 80 includes a first ReRAM device 12 having a first endcoupled to bitline BL 16. A p-channel transistor 82 is coupled in serieswith a second end of first ReRAM device 12 and has its source connectedto the second end of ReRAM device 12. The drain of p-channel transistor82 is connected to a switch node 20.

A second ReRAM device 14 has a first end coupled to bitline BL! 18. Ann-channel transistor 84 is coupled in series with a second end of secondReRAM device 14 and has its source connected to the second end of ReRAMdevice 14. The drain of n-channel transistor 84 is connected to theswitch node 20. Persons of ordinary skill in the art will appreciatethat the value of the potentials applied to (BL) 16 and (BL!) 18 will beselected as a function of the particular feature size and other aspectsof the technology employed. Typical operating voltages that are appliedto (BL) 16 and (BL!) 18 are 1.5V and 0V, respectively.

The gate of p-channel transistor 82 is connected to bias voltage node 86to bias the gate with respect to its source, The gate of n-channeltransistor 84 is connected to bias voltage node 88 to bias the gate withrespect to its source. The bias voltages for the p-channel transistor 82and n-channel transistor 84 to be applied to the respected bias voltagenodes 86 and 88 should be respectively chosen at design time to set theconductance of each of the p-channel and n-channel transistors so thateach of ReRAM device 12 and ReRAM device 14 will have a voltagepotential thereacross of between 0.25V to 0.5V more or less when it isin its “off” state. This bias voltage is nominally about 0.75V withrespect to the source of the transistor for the voltages of 1.5V and 0Vmentioned above, but, as noted, will vary given individual devices andprocesses.

A programming transistor 28 has a gate coupled to a word line (WL) 30.The drain of programming transistor 28 is connected to switch node 20and its source is connected to a word line source (WLS) 32.

Selecting the voltage to which the ReRAM devices will be subjected intheir “off” states involves an engineering tradeoff between the leakageof the off-state ReRAM device and the subthreshold conduction of theseries transistor 82 or 84. The voltage selected in any particularsituation will depend on the sizes and geometries of the ReRAM andtransistor devices used, and the fabrication process employed. As noted,typical design tradeoffs should result in a voltage across the off-stateReRAM device in the range of 0.25V to 0.5V more or less when usingoperating voltages in the neighborhood of 1.5V. Higher voltages acrossthe off-state ReRAM device will exponentially increase the ReRAM deviceleakage, and lower voltages will drop more of the off-state voltageacross the series p-channel or n-channel transistor, causing moresubthreshold conduction through the transistor. In a typical design, acurrent flow of no more than 100 pA more or less should be flowingthrough the off-state ReRAM device.

FIG. 3B is a plot showing the current vs. voltage (I-V) characteristicsof an n-channel MOS transistor having a V_(t) of 0.5V, with its gate at0.75V and operating in its subthreshold region superimposed on a plot ofthe leakage current of an “off” state ReRAM device. FIG. 3B shows anillustrative method for selecting the “off” state voltage of a ReRAMdevice in accordance with the present invention whereby the I-Vcharacteristics of the ReRAM device and the transistor intersect at aselected ReRAM leakage current of 100 pA at a voltage of 0.5V and atransistor having a 0.5V V_(t). Persons of ordinary skill in the artwill readily understand that a similar plot can easily be developed fora p-channel transistor to help select an operating point.

In operation, as indicated above, one of ReRAM devices 12 and 14 will beset to its “on” state and the other will be set to its “off” state.Depending on which one of the ReRAM devices 12 and 14 is “on” and whichone is “off” switch node 20 will either be pulled up to the voltage onBL 16 or pulled down to the voltage on BL! 18.

The gate of a switch transistor 22 is coupled to switch node 20. Thedrain of the switch transistor is connected to a first programmable node24 and the source of the switch transistor is connected to a secondprogrammable node 26. The first programmable node 24 can be connected tothe second programmable node 26 by turning on the switch transistor 22.

If ReRAM device 12 is in its “on” state and ReRAM device 14 is in its“off” state, switch node 20 is pulled up to the voltage on BL 16, andswitch transistor 22 will be turned on, if ReRAM device 12 is in its“off” state and ReRAM device 14 is in its “on” state, switch node 20 ispulled down to the voltage on BL! 18, and switch transistor 22 will beturned off. Persons of ordinary skill in the art will note that theentire potential between one of (BL) 16 and (BL!) 18 and the switch node20 will not exist across the “off” state ReRAM device but will be sharedacross the one of ReRAM devices 12 and 14 that is in the “off” state andthe one of p-channel transistor 82 and n-channel transistor 84 connectedbetween it and the one of bitlines (BL) 16 and (BL!) 18 with which it isassociated.

The offstate leakage in some ReRAM devices is a strong function of thereverse or “off” state voltage applied to the structure. The p-channeltransistor 82 and n-channel transistor 84 serve to reduce the offstatevoltage across the ReRAM devices to which they are directly connected tominimize the leakage. When ReRAM device 12 is in its “off” state andReRAM device 14 is in its on state, the source of n-channel transistor84 is at the voltage on bitline BL! (e.g., 0V) and its gate is at thevoltage provided at bias voltage node 88, which for ease ofunderstanding is shown as an exemplary 0.75V. Under these conditions,n-channel transistor 84 is turned on hard because its gate-to-sourcevoltage is at the voltage of bias voltage node 88, and switch node 20 ispulled down to the voltage of bitline BL! (e.g., 0V). As a result,switch node 20 is pulled down to the voltage at bitline BL! (e.g., 0V).The gate of p-channel transistor 82 is at the voltage of bias voltagenode 86, shown as an exemplary 0.75V. The source of p-channel transistor84 is at around 1V more or less because the 1.5V between the bitline BL16 and the switch node 20 is divided across the off-state ReRAM device12 and the p-channel transistor 82 such that no more than about 0.25 to0.5V more or less is across ReRAM device 12 to maintain the leakagecurrent at, or below, the target maximum. Under these conditions thegate-to-source voltage of p-channel transistor 82 is only about −0.25Vmore or less and p-channel transistor 82 is operating in itssubthreshold region as explained above in relation to FIG. 3B.

When ReRAM device 12 is in its on state and ReRAM device 14 is in itsoff state, the source of p-channel transistor 82 is at the voltage onbitline BL (e.g., 1.5V) and its gate is at the voltage provided at biasvoltage node 88, which for ease of understanding is shown as anexemplary 0.75V. Under these conditions, p-channel transistor 82 isturned on hard because its gate-to-source voltage is −0.75V, i.e. thevoltage of bias voltage node 88 less the voltage at bitline BL 16 (e.g.1.5V), and switch node 20 is pulled up to the voltage on bitline BL. Thegate of n-channel transistor 84 is at the voltage of bias voltage node88, shown as an exemplary 0.75V. The source of n-channel transistor isat 0.5V more or less because the 1.5V between the bitline BL! 18 (0V)and the switch node 20 (1.5V) is divided across the off-state ReRAMdevice 14 and the n-channel transistor 84 such that only about 0.25 to0.5V more or less is across ReRAM device 14 to maintain the leakagecurrent at, or below, the target maximum. Under these conditions thegate-to-source voltage of n-channel transistor 84 is only around 0.25Vmore or less and n-channel transistor 84 is operating in itssubthreshold region as explained above in relation to FIG. 3B.

The bias voltage for bias voltage nodes 86 and 88 are set responsive tothe inherent off-state resistance of the associated ReRAM device 12 or14 so as to achieve the desired off-state potential thereacross,particular such that only about 0.25V to 0.5V, more or less, is acrossReRAM device 12 or 14.

Persons of ordinary skill in the art will appreciate that the voltagevalues used in the above description of the operation of the ReRAMmemory cell 80 are nominal values presented for the purposes ofillustration.

Referring now to FIGS. 4A and 4B together, a cross-sectional view inFIG. 4A and a top view in FIG. 4B together show an illustrativesemiconductor layout for a ReRAM cell 80 like that shown in FIG. 3A.Persons of ordinary skill in the art will appreciate that the layoutshown in FIG. 4A and FIG. 4B is only one non-limiting example and thatother layouts of the ReRAM cell of the present invention are possibleand are intended to be within the scope of the present invention.

ReRAM cell 80 is formed in a semiconductor substrate 90, which, asunderstood by persons of ordinary skill in the art, could be a substrateor a well region formed within a semiconductor substrate on t which theintegrated circuit containing it is fabricated. In the example shown inFIG. 4A, the substrate 90 is a p-type substrate.

ReRAM devices 12 and 14 are connected as a push-pull ReRAM cell. TheReRAM device 12 is formed on a segment 92 of a lower metal layer in theintegrated circuit. The other end of ReRAM device 12 is connected to abitline (BL) formed from a segment 94 of an upper metal layer by aninter-metal via 96. In the embodiment shown in FIG. 4A the upper andlower metal layers are a first metal interconnect layer (M1) and asecond metal interconnect layer (M2) although persons of ordinary skillin the art will understand that this is not critical to the invention.

The transistors for the ReRAM cell 80 are formed in the substrate 90 andare separated from one another by isolations regions, shown as shallowtrench isolation (STI) regions 98. P-channel transistor 82 is formed inn-well 100 (which is biased at the highest voltage in the circuit so asto never become forward biased) and has a source 102 connected to themetal segment 92 by via 104. The drain 106 of p-channel transistor 82 isconnected to a segment 108 of the lower metal layer by a via 110.Segment 108 forms a part of the switch node 20 (FIG. 3A) of the ReRAMcell 90. The gate of p-channel transistor 82 is formed from polysiliconline 112 that is connected to a Bias 1 node to bias p-channel transistor82 at about 0.75 V with respect to its source when the ReRAM memory cellis operated at a supply voltage of about 1.5V, as described above.

The ReRAM device 14 is formed on a segment 114 of the lower metal layer(M1) in the integrated circuit. The other end of ReRAM device 14 isconnected to a bitline (BL!) formed from a segment 116 of the uppermetal layer (M2) by an inter-metal via 118.

Segment 114 is connected to the source 120 of the n-channel transistor84 by a via 122. The drain 124 of the n-channel transistor 84 isconnected to the segment 108 by via 126. The gate of n-channeltransistor 84 is formed from a polysilicon line 128 that is connected toa Bias 2 node to bias n-channel transistor 84 at about 0.75 V withrespect to its source when the ReRAM memory cell is operated at a supplyvoltage of about 1.5V, as described above.

The switch transistor 22 of ReRAM cell 80 has a channel region 130underneath its gate which is formed from a polysilicon segment 132.Polysilicon segment 132 is connected to metal segment 134 by via 136. Asshown in FIG. 4B, segment 134 is connected to segment 108 and may bepart of a single metal line.

N-channel programming transistor 28 has its source 138 connected tometal segment 140 by a via 142, serving as the word line source (WLS) 32of the ReRAM cell 80. The drain 144 of programming transistor 28 isconnected to metal segment 146 by a via 148. As shown in FIG. 4B,segment 146 is also connected to segments 108 and 134 and may be part ofa single metal line 150 that constitutes the switch node 20. The gate ofn-channel programming transistor 28 is formed from polysilicon line 152that forms the word line 30 of the n-channel programming transistor 28.

Referring now to FIG. 5, a schematic diagram depicts four illustrativeReRAM cells in an array to show a method for programming and erasing theReRAM cells. The cells are identified by row and column location, R1C1being the cell in the first row and first column, R1C2 being the cell inthe first row second column, R2C1 being the cell in the second row andfirst column, and R2C2 being the cell in the second row second column.

The table of FIG. 6 shows the voltages to apply to the column lines, bitlines and word lines to perform the operations associated with eachcolumn of the table. The reference numeral designations used for theelements in FIG. 5 are the reference numerals used for these elements inFIG. 3A, followed by -x-y where x is the row of the array containing theelement and y is the column of the array containing the element.

The voltages listed in FIG. 6 are nominal values and may vary indifferent designs as a function of the technology used. For example,2.5V is applied to one of WL1 and WL2 for certain operations. Thevoltage actually necessary to perform these operations depends on theV_(t) of the programming transistors 28 (e.g., about 0.4V) and willtherefore normally be less than 2.5V, but 2.5V is chosen because it is avoltage that usually present anyway in the integrated circuit and so isa convenient choice. The same is true for the 1.8V voltage values, whichare normally present in integrated circuits, 1.8V being a typicalvoltage available to overdrive transistor gates to eliminate the V_(t)voltage drop across a turned on transistor.

Before programming any of the ReRAM cells, they are all erased byplacing both of the ReRAM devices in the ReRAM cells to their “off”states.

Column A represents the voltages applied to erase (turn off) all upperReRAM devices in the cells. When the voltages listed in column A of thetable are applied to the array of FIG. 5, each of the programingtransistors 28-1-1, 28-1-2, 28-2-1, and 28-2-2 in the four ReRAM memorycells R1C1, R1C2, R2C1, and R2C2 has 0V on its source and 1.8V on itsgate and is turned on, placing each switch node 22-1-1, 22-1-2, 22-2-1,and 22-2-2 at 0V. The upper bitlines BL1 and BL2 each have 1.8V on them.Thus, the upper ReRAM devices 12-1-1, 12-1-2, 12-2-1, and 12-2-2 eachhave 1.8V across them, allowing current to flow through them to drawions out of the electrolyte layer back to the ion source layer. Thelower bitlines BL1! and BL2! each have 0V on them. Thus, the lower ReRAMdevices 14-1-1, 14-1-2, 14-2-1, and 14-2-2 each have 0V across them,thus not allowing any current to flow through them.

Column B represents the voltages applied to erase all lower ReRAMdevices in the cells. When the voltages listed in column B of the tableare applied to the array of FIG. 5, each of the programing transistors28-1-1, 28-1-2, 28-2-1, and 28-2-2 in the four ReRAM memory cells R1C1,R1C2, R2C1, and R2C2 has 1.8V on its source and 2.5V on its gate and isturned on, placing each switch node 22-1-1, 22-1-2, 22-2-1, and 22-2-2at 1.8V. The lower bitlines BL1! and BL2! each have 0V on them. Thus,the lower ReRAM devices 14-1-1, 14-1-2, 14-2-1, and 14-2-2 each have1.8V across them, allowing current to flow through them to draw ions outof the electrolyte layer back to the ion source layer. The upperbitlines BL1 and BL2 each have 1.8V on them. The upper ReRAM devices12-1-1, 12-1-2, 12-2-1, and 12-2-2 thus each have 0V across them, notallowing any allowing current to flow through them.

Once all of the ReRAM cells have been erased, each ReRAM cell may beprogrammed to turn it “on” thereby turning on its associated switchtransistor or to turn it “off” thereby turning off its associated switchtransistor. As described below, the programming is accomplishedresponsive to the proper bias of the bitlines, word lines and respectiveprogramming transistor.

Column C represents the voltages applied to turn on the ReRAM cell atR1C1 by turning on the upper ReRAM device 12-1-1 in that cell to pull upthe switch node to turn on the switch transistor. When the voltageslisted in column C of the table are applied to the array of FIG. 5,programming transistor 28-1-1 has 1.8V on its source, 2.5V on its gateand will be turned on, driving the switch node 22-1-1 in ReRAM cell R1C1to 1.8V. Bitline BL1 has 0V on it, and ReRAM device 12-1-1 willtherefore have a voltage of 1.8V across it, the bottom end being morepositive than the top end. This is the condition for programming ReRAMdevice 12-1-1. ReRAM device 14-1-1 will also have a voltage of 1.8Vacross it, but the bottom end is more negative than the top end andtherefore ReRAM device 14-1-1 will not be programmed.

Programming transistor 28-1-2 has 0V on its source, 2.5V on its gate andwill be turned on, driving the switch node 22-1-2 in ReRAM cell R1C2 to0V. Because BL2 and BL2! both have 0V on them, both ReRAM devices 12-1-2and 14-1-2 in ReRAM cell R1C2 will have 0V across them and will not beprogrammed.

Programming transistors 28-2-1 and 28-2-2 each has 0V on its gate andwill be turned off. The switch nodes 22-2-1 and 22-2-2 in ReRAM cellsR2C1 and R2C2 will be either floating or at the potential on bothbitlines BL2 and BL2! if one of the ReRAM devices in those cells hasbeen programmed. Since bitlines BL2 and BL2! are both at 0V, no ReRAMdevices in cells R2C1 and R2C2 in the second row of the array will beprogrammed.

Column D represents the voltages applied to turn off the ReRAM cell atR1C1 by turning on the lower ReRAM device 14-1-1 in that cell to pulldown the switch node 22-1-1 to turn off the associated switchtransistor. When the voltages listed in column D of the table areapplied to the array of FIG. 5, programming transistor 28-1-1 has 0V onits source, 2.5V on its gate and will be turned on, driving the switchnode 22-1-1 in ReRAM cell R1C1 to 0V. Bitline BL!1 has 1.8V on it, andReRAM device 14-1-1 will have a voltage of 1.8V across it, the bottomend being more positive than the top end. This is the condition forprogramming ReRAM device 14-1-1. ReRAM device 12-1-1 will also have avoltage of 1.8V across it, but the bottom end is more negative than thetop end and therefore ReRAM device 12-1-1 will not be programmed.

Programming transistor 28-1-2 has 1.8V on its source, 2.5V on its gateand will be turned on, driving the switch node 22-1-2 in ReRAM cell R1C2to 1.8V. Because BL2 and BL2! both have 1.8V on them, both ReRAM devices12-1-2 and 14-1-2 in ReRAM cell R1C2 will have 0V across them and willnot be programmed.

Programming transistors 28-2-1 and 28-2-2 each has 0V on its gate andwill be turned off. The switch nodes 22-2-1 and 22-2-2 in ReRAM cellsR2C1 and R2C2 will be either floating or at the potential on bothbitlines BL2 and BL2! if one of the ReRAM devices in those cells hasbeen programmed. All bitlines are at 1.8V, but because the switch nodes22-2-1 and 22-2-2 are either floating or at the potential of bitlinesBL2 and BL!2, no ReRAM devices in cells R2C1 and R2C2 in the second rowof the array will be programmed.

Column E represents the voltages applied to turn on the ReRAM cell atR1C2 by turning on the upper ReRAM device 12-1-2 in that cell to pull upthe switch node 22-1-2 to turn on the associated switch transistor. Theconditions are similar to those for column C, except that the source ofprogramming transistor 28-1-2 is now at 1.8V and is turned on (and thesource of transistor 28-1-1 is now at 0V) and ReRAM device 12-1-2 isprogrammed because it has 0V at its top end and 1.8V on its bottom end.ReRAM device 14-1-2 will also have a voltage of 1.8V across it, but itsbottom end is more negative than its top end and therefore ReRAM device14-1-2 will not be programmed. Persons of ordinary skill in the art willappreciate that the ReRAM cells in the second row of the array are notprogrammed for the reasons set forth in the explanation of the column Cconditions.

Column F represents the voltages applied to turn off the ReRAM cell atR1C2 by turning on the lower ReRAM device 14-1-2 in that cell to pulldown the switch node 22-1-2 to turn off the associated switchtransistor. When the voltages listed in column F of the table areapplied to the array of FIG. 5, programming transistor 28-1-2 has 0V onits source, 2.5V on its gate and will be turned on, driving the switchnode 22-1-2 in ReRAM cell R1C2 to 0V. Bitline BL2! has 1.8V on it, andReRAM device 14-1-2 will have a voltage of 1.8V across it, since bitlineBL2 has 1.8V on it, the bottom end being more positive than the top end.This is the condition for programming ReRAM device 14-1-2. ReRAM device12-1-2 will also have a voltage of 1.8V across it, but the bottom end ismore negative than the top end and therefore ReRAM device 12-1-2 willnot be programmed.

Programming transistor 28-1-1 has 1.8V on its source, 2.5V on its gateand will be turned on, driving the switch node 22-1-1 in ReRAM cell R1C1to 1.8V. Because BL1 and BL1! both have 1.8V on them, both ReRAM devices12-1-1 and 14-1-1 in ReRAM cell R1C1 will have 0V across them and willnot be programmed.

Programming transistors 28-2-1 and 28-2-2 each has 0V on its gate andwill be turned off. The switch nodes 22-2-1 and 22-2-2 in ReRAM cellsR2C1 and R2C2 will be either floating or at the potential on bothbitlines BL1 and BL1! if one of the ReRAM devices in those cells hasbeen programmed. All bitlines are at 1.8V, but because the switch nodes22-2-1 and 22-2-2 are either floating or at the potential of bitlinesBL1 and BL2!, no ReRAM devices in cells R2C1 and R2C2 in the second rowof the array will be programmed.

Column G represents the voltages applied to turn on the ReRAM cell atR2C1 by turning on the upper ReRAM device 12-2-1 in that cell to pull upthe switch node 22-2-1 to turn on the associated switch transistor.Column H represents the voltages applied to turn off the ReRAM cell atR2C1 by turning on the lower ReRAM device 14-2-1 in that cell to pulldown the switch node 22-2-1 to turn off the associated switchtransistor. Column I represents the voltages applied to turn on theReRAM cell at R2C2 by turning on the upper ReRAM device 12-2-2 in thatcell to pull up the switch node 22-2-2 to turn on the associated switchtransistor. Column J represents the voltages applied to turn off theReRAM cell at R2C1 by turning on the lower ReRAM device 14-2-2 in thatcell to pull down the switch node 22-2-1 to turn off the associatedswitch transistor. From the conditions described with reference tocolumns C through F for programming the ReRAM cells in the first row ofthe array to either their “on” or “off” states, persons of ordinaryskill in the art will readily appreciate from FIG. 5 and FIG. 6 how theprogramming of ReRAM cells R2C1 and R2C2 in the second row of the arrayis accomplished.

Referring now to FIG. 7A, a schematic diagram shows another illustrativeembodiment of a push-pull ReRAM cell 160 in accordance with the presentinvention. The ReRAM cell 160 is similar to the ReRAM cell 80 of FIG. 3Aand reference numerals designating features of ReRAM cell 160 that arethe same as features in the ReRAM cell 80 of FIG. 3A will be designatedusing the same reference numerals used in FIG. 3A.

ReRAM cell 160 includes a first ReRAM device 12 having a first endcoupled to bitline BL 16. A p-channel transistor 82 is coupled in serieswith a second end of first ReRAM device 12 and has its source connectedto the second end of ReRAM device 12. The drain of p-channel transistor82 is connected to a switch node 20.

A second ReRAM device 14 has a first end coupled to bitline BL! 18. Ann-channel transistor 84 is coupled in series with a second end of secondReRAM device 14 and has its source connected to the second end of ReRAMdevice 14. The drain of n-channel transistor 84 is connected to theswitch node 20. Persons of ordinary skill in the art will appreciatethat the value of the potentials applied to (BL) 16 and (BL!) 18 will beselected as a function of the particular feature size and other aspectsof the technology employed. Typical operating voltages that are appliedto (BL) 16 and (BL!) 18 are 1.5V and 0V, respectively.

The gate of p-channel transistor 82 is connected to bias voltage node 86to bias the gate with respect to its source. The gate of n-channeltransistor 84 is connected to bias voltage node 88 to bias the gate withrespect to its source. As with the ReRAM cell 80 of FIG. 3A, the biasvoltages for the p-channel transistor 82 and n-channel transistor 84 tobe applied to the respected bias voltage nodes 86 and 88 should berespectively chosen at design time to set the conductance of each of thep-channel and n-channel transistors so that each of ReRAM device 12 andReRAM device 14 will have a voltage potential thereacross so as toachieve the target maximum “off”-stage leakage current, the voltagepotential being between 0.25V to 0.5V more or less at the target “off”state current leakage when it is in its “off” state, This bias voltageis nominally about 0.75V with respect to the source of the transistorfor the voltages of 1.5V and 0V mentioned above, but, as noted, willvary given individual devices and processes and target maximum“off”-stage leakage current.

The difference between ReRAM cell 160 and ReRAM cell 80 of FIG. 3A isthat in the ReRAM cell 80 of FIG. 3A, a single programming transistor 28has a gate coupled to a word line (WL) 30, the drain of programmingtransistor 28 is connected to switch node 20 and its source is connectedto a word line source (WLS) 32. In the ReRAM cell 160 of FIG. 7A, a pairof programming transistors 28 a and 28 b are employed. As illustrated inFIG. 7A, programming transistor 28 a is an n-channel transistor havingits gate coupled to a word line (WL) 30, its drain coupled to the secondend of ReRAM device 12, and its source coupled to a word line source(WLS) 32. Programming transistor 28 b is an n-channel transistor havingits gate coupled to the word line (WL) 30, its drain coupled to thesecond end of ReRAM device 14, and its source coupled to the word linesource (WLS) 32. Other variations of this circuit are intended to fallwithin the scope of the present invention, and include embodiments whereprogramming transistors 28 a and 28 b are both p-channel transistors (asillustrated in FIG. 7B), where programming transistor 28 a is ap-channel transistor and programming transistor 28 b is an n-channeltransistor (as illustrated in FIG. 7C), and where programming transistor28 a is an n-channel transistor and programming transistor 28 b is ap-channel transistor (as illustrated in FIG. 7D).

ReRAM cell 160 is larger than ReRAM cell 80, as it requires anotherprogramming transistor, but has the advantage that the cell can beprogrammed and erased using a current path that does not include thep-channel and n-channel bias transistors 82 and 84, and switch node 20,allowing higher currents to be used during programing. The bias ofswitch node 20 is irrelevant during programming and erasing. Duringprogramming, the cell to be programmed or erased is selected using theWL and WLS lines 30 and 32, and the choice of which of ReRAM devices 12and 14 is to be programmed or erased is selected by applying theappropriate voltages to bitlines BL 16 and BL! 18.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications than mentioned above are possible without departingfrom the inventive concepts herein. The invention, therefore, is not tobe restricted except in the spirit of the appended claims.

What is claimed is:
 1. A low-leakage resistive random access memory(ReRAM) cell comprising: a complementary pair of bit lines; a switchnode; a first ReRAM device having a first end connected to a first oneof the bit lines; a p-channel transistor having a source connected to asecond end of the first ReRAM device, a drain connected to the switchnode, and a gate connected to a bias potential; a second ReRAM devicehaving a first end connected to a second one of the hit lines; and ann-channel transistor having a source connected to a second end of thesecond ReRAM device, a drain connected to the switch node, and a gateconnected to a bias potential.
 2. The ReRAM cell of claim 1, furthercomprising a programming transistor having a drain connected to theswitch node, a source connected to a source word line and a gateconnected to a word line.
 3. The ReRAM cell of claim 1 further includinga switch transistor having a gate connected to the switch node, a sourceconnected to a first programmable node and a drain connected to a secondprogrammable node.
 4. The ReRAM cell of claim 1 wherein the first andsecond bias potentials are equal.
 5. The ReRAM cell of claim 4 whereinthe first and second bias potentials are one half of an operatingvoltage between the complementary pair of bitlines.
 6. The ReRAM cell ofclaim 1 wherein the first and second ReRAM devices are formed between alower metal interconnect line and an upper metal interconnect line in anintegrated circuit.
 7. The ReRAM cell of claim 6 wherein lower metalinterconnect line is a first metal interconnect line and the upper metalinterconnect line is a first metal interconnect line is a second metalinterconnect line.
 8. The ReRAM cell of claim 1, further comprising: afirst programming transistor having a drain connected to the second endof the first ReRAM device, a source connected to a source word line anda gate connected to a word line; and a second programming transistorhaving a drain connected to the second end of the second ReRAM device, asource connected to the source word line and a gate connected to theword line.